In wireless communication systems, downlink reference signals are typically created to provide a reference for channel estimation used in coherent demodulation as well as a reference for a channel quality measurement used in multi-user scheduling. In the LTE Rel-8 specification, one single type of downlink reference format called a cell-specific reference signal (CRS) is defined for both channel estimation and channel quality measurement. The characteristics of Rel-8 CRS include that, regardless of the multiple in multiple out (MIMO) channel rank that the user equipment (UE) actually needs, the base station can always broadcast the CRS to all UE based on the largest number of MIMO layers/ports.
In the 3GPP LTE Rel-8 system, the transmission time is partitioned into units of a frame that is 10 ms long and is further equally divided into 10 subframes, which are labeled as subframe #0 to subframe #9. While the LTE frequency division duplexing (FDD) system has 10 contiguous downlink subframes and 10 contiguous uplink subframes in each frame, the LTE time-division duplexing (TDD) system has multiple downlink-uplink allocations, whose downlink and uplink subframe assignments are given in Table 1, where the letters D, U and S represent the corresponding subframes and respectively refer to the downlink subframe, uplink subframe and special subframe that contains the downlink transmission in the first part of a subframe and the uplink transmission in the last part of subframe.
TABLE 1TDD allocation configurationsUplink-downlinkDownlink-to-UplinkSubframe numberconfigurationSwitch-point periodicity012345678905 msDSUUUDSUUU15 msDSUUDDSUUD25 msDSUDDDSUDD310 ms DSUUUDDDDD410 ms DSUUDDDDDD510 ms DSUDDDDDDD65 msDSUUUDSUUD
In one system configuration instance (called normal cyclic prefix, or normal-CP) in LTE, each subframe includes 2NsymbDL=14 equal-duration time symbols with the index from 0 to 13. In another system configuration instance (called extended cyclic prefix, or extended-CP) in LTE, each subframe contains 2NsymbDL=12 equal-duration time symbols with an index from 0 to 11.
The frequency domain resource is partitioned into subcarriers up to the full bandwidth within one time symbol. One physical resource block (PRB) is defined over a rectangular 2-D frequency-time resource area covering 12 contiguous subcarriers over the frequency domain and 1 subframe over the time domain wherein the PRB holds 12*14=168 resource elements (RE) for a normal-CP subframe as shown in FIG. 2, for example. FIG. 3 illustrates 12*12=144 REs for an exemplary extended-CP subframe.
In addition, each subframe can also contain two equal-length slots. Each slot may contain 7 OFDM (orthogonal frequency-division multiplexing) symbols. In normal-CP configuration, the OFDM symbols are indexed per slot, where the symbol index runs from 0 to 6; the OFDM symbols can be also indexed per subframe, where the symbol index runs from 0 to 13.
Each regular subframe is partitioned into two parts: the PDCCH (Physical Downlink Control Channel) region and the PDSCH (Physical Downlink Shared Channel) region. The PDCCH region normally occupies the first several symbols per subframe and carries the handset specific control channels, and the PDSCH region occupies the rest of the subframe and carries the general-purpose traffic. The LTE system requires the following mandatory downlink transmissions:                Primary synchronization signal (PSS) and secondary synchronization signal (SSS): These two signals repeat in every frame and serve for the initial synchronization and cell identification detection after UE powers up. The transmission of PSS occurs at symbol #6 in subframes {0,5} for FDD systems with normal-CP, and at symbol #2 in subframes {1,6} for TDD systems; the transmission of SSS occurs at symbol #5 in subframes {0,5} for FDD with normal-CP, and at symbol #13 in subframes {0,5} for TDD with normal-CP;        Physical broadcast channel (PBCH): PBCH also repeats in every frame, and serves for broadcasting of essential cell information. Its transmission occurs over 4 symbols {7˜10} in subframe #0;        System information block (SIB): SIB is the broadcast information that is not transmitted over PBCH. It is carried in a specific PDSCH that is decoded by every handset. There are multiple types of SIB in LTE, most of which have a configurably longer transmission cycle, except SIB type-1 (SIB1). SIB1 is fix-scheduled at subframe #5 in every even frame. SIB is transmitted in PDSCH identified by a system information radio network temporary identifier (SI-RNTI) given in the corresponding PDCCH;        Paging channel (PCH): The paging channel is used to address the handset in idle mode or to inform the handset of a system-wide event, such as the modification of content in SIB. In LTE Rel-8, PCH can be sent in any subframe from a configuration-selective set from {9}, {4,9} and {0, 4, 5, 9} for FDD and {0}, {0,5}, {0, 1, 5, 6} for TDD. PCH is transmitted in PDSCH identified by the paging RNTI (P-RNTI) given in the corresponding PDCCH; and        Cell-specific reference signal (CRS): CRS serves for downlink signal strength measurement, and for coherent demodulation of PDSCH in the same resource block. CRS is also used for the verification of cell identification done on PSS and SSS. CRS transmissions have the same pattern in each regular subframe, and occur on symbols {0, 1, 4, 7, 8, 11} with a maximum of four transmission antenna ports in a normal-CP subframe and symbols {0, 1, 3, 6, 7, 9} in an extended-CP subframe. Each CRS symbol carries two CRS subcarriers per port per resource block dimension in frequency domain, as shown in FIGS. 2 and 3. The actual subcarrier index of CRS is shifted by vshift=NIDcell mod 6, where NIDcell is the cell identification. LTE Rel-8 also defines a type of UE specific reference signal (URS) on the antenna port 5. There are 12 URS REs per PRB, occupying 4 symbols in a normal-CP subframe as shown in FIG. 2, and 3 symbols in an extended-CP subframe as shown in FIG. 3. The actual subcarrier index of URS is shifted by vshift=NIDcell mod 3. Although CRS is allocated across the full bandwidth, URS is assigned on a per PRB basis. FIGS. 2 and 3 show examples of CRS and URS with vshift=0.        
As 3GPP LTE evolves from Rel-8 to Rel-10 (also called LTE-advance or LTE-A), due to the large number of supported antenna ports (up to 8), it can cost a large amount of overhead to maintain the CRS-like reference signal on all ports. Downlink reference signal roles can be separated into the following different RSs:                Demodulation Reference Signal (DMRS): this type of RS is used for coherent channel estimation and should have sufficient density and should be sent on a per UE basis; and        Channel State Information Reference Signal: this type of RS is used for coherent channel estimation and should have sufficient density and should be sent on a per UE basis.        
According to the 3GPP standard body:                P DMRS can be assigned on a PRB basis, and a DMRS pattern in each PRB can be located at 24 fixed REs in a normal-CP subframe as shown in FIG. 2 or 16 fixed REs in an extended-CP subframe as shown in FIG. 3.        CSI-RS is allocated across the whole system bandwidth. NANTε{2, 4, 8} is a number of CSI-RS antenna ports per cell. Note that the number of CSI-RS antenna ports is also referred to as NCSIRS in this application. Both NANT and NCSIRS are inter-changeable in the following description of this application. Then in each PRB, there are NANT CSI-RS REs labeled as {0, 1, . . . NANT−1}, of which every two CSI-RS REs indexed by 2j and 2j+1 are code-multiplexed.        CSI-RS allocation with NANT=8 (8-Tx) in a normal-CP subframe is shown in FIG. 2, where FIG. 2(a) shows the CSI-RS reuse patterns that cannot coexist with port-5 URS, and FIG. 2(b) shows the CSI-RS reuse patterns that can coexist with port-5 URS. The CSI-RS reuse patterns in FIG. 2(a) can be applied in both frame structure 1 (FS-1 i.e. FDD) and frame structure 2 (FS-2 i.e. TDD), while the C CSI-RS reuse patterns in FIG. 2(b) can be applied in FS-2 (TDD) only.        The CSI-RS allocation with NANT=8 (8-Tx) in extended-CP subframe is shown in FIG. 3, where FIG. 3(a) shows the CSI-RS reuse patterns that cannot coexist with port-5 URS, and FIG. 3(b) shows the CSI-RS reuse patterns that can coexist with port-5 URS. The CSI-RS reuse patterns in FIG. 3(a) can be applied in both frame structure 1 (FS-1 i.e. FDD) and in frame structure 2 (FS-2 i.e. TDD). The CSI-RS reuse patterns in FIG. 3(b) can be applied in FS-2 (TDD) only.        For NANT={2, 4} (2-Tx and 4-Tx), the CSI-RS RE locations are nested inside the 8-Tx CSI-RS RE locations. When NANT=2, the 2-Tx CSI-RS reuse pattern identified by RE#<0, 1> can be mapped to any REs labeled with <2j, 2j+1> in any 8-Tx reuse pattern. When NANT=4, the 4-Tx CSI-RS reuse pattern identified by RE#<0, 1, 2, 3>can be mapped to any REs labeled with <4j, 4j+1, 4j+2, 4j+3> in any 8-Tx reuse pattern.        
CSI-RS is transmitted not only for the intra-cell measurement to support MIMO transmission with up to eight antenna ports in the serving cell, but also for inter-cell measurement in the coordinate-multiple-point (CoMP) transmission, in which the user equipment (UE) or mobile station needs to measure the CSI-RS transmitted from the base stations in surrounding cells and then to report those measurements to the serving cell. All the cells whose CSI-RSs need to be measured by a UE construct the measurement set for that UE.
However, it is not always possible for the UE to measure the signals originated from the non-serving cells because there could be interference between those signals and strong signals transmitted in the serving cell if the cells work on the same frequency. In order to maintain the measurement quality on inter-cell CSI-RS, it is proposed in 3GPP LTE that the PDSCH REs that occupy the same RE locations used by surrounding cells to transmit CSI-RS be muted (transmitted with zero power).
Further, partial muting was also proposed to mute only some of PDSCH REs that collide with CSI-RS REs in surrounding cells wherein the transmission is not muted the on the rest of the REs that collide with CSI-RS REs in surrounding cells. This was to provide better trade-offs between CoMP performance and muting overhead and to provide a flexible adjustment mechanism based on the actual volume of CoMP traffic in the system.
However, the prior art does not provide for the configuration and transmission of CSI-RS signals (i.e., how to inform the UE of the CSI-RS RE locations in the measurement set). In addition, there is a further need to configure the CSI-RS related muting.